How Much Attic Ventilation Do I Need?
Quick answer: use the 1/300 rule — 1 square foot of net free ventilation area per 300 square feet of attic floor. A 1,200 sq ft attic needs 4 sq ft of ventilation: 2 sq ft of intake at the soffits and 2 sq ft of exhaust at the ridge. No vapor barrier? Size to 1/150 instead (double the venting).
- The 1/300 rule explained
- Net free area (NFA) — what actually counts
- Intake and exhaust balance
- Vent types and their NFA
- Worked example: 1,200 sq ft attic
- Common mistakes
- FAQ
The 1/300 rule explained
US building codes size attic ventilation on the 1/300 rule: for every 300 square feet of attic floor area, provide 1 square foot of net free ventilation area (NFA). An attic above a 1,200 sq ft house floor → 1,200 ÷ 300 = 4 sq ft of ventilation. When there's no vapor barrier on the ceiling below, codes tighten to 1/150 — twice the venting. Many pros size everything to 1/150 anyway, because more airflow means cooler summers and drier winters.
Net free area (NFA) — what actually counts
Net free area is the open area a vent actually passes air through — not the size of the vent cover. A 16×8 inch soffit vent with louvers might deliver only 56 square inches of NFA. Always use the NFA number printed on the vent (or its spec sheet), and use the same units as your attic calculation: 144 sq in = 1 sq ft.
Intake and exhaust balance
Ventilation only works when air enters low and exits high. Split the total 50% intake / 50% exhaust: intake goes in soffit or eave vents, exhaust goes at the ridge or high in the roof. A 4 sq ft total means 2 sq ft of soffit NFA plus 2 sq ft of ridge NFA. Venting the ridge without soffit intake just depressurizes the attic and pulls air from the living space.
Vent types and their NFA
| Vent type | Typical NFA | Where it goes |
|---|---|---|
| Ridge vent | ~18 sq in per linear ft | Peak of the roof (exhaust) |
| Soflit / eave vent | ~5–7 sq in per ft (continuous) | Underside of the eaves (intake) |
| Round soffit vent (4 in) | ~10 sq in each | Eaves (intake) |
| Roof louvers / box vents | ~50–75 sq in each | Upper roof (exhaust) |
| Gable vents (18×24 in) | ~230 sq in each | Gable ends (passive, mixed) |
Worked example: a 1,200 sq ft attic
- Total NFA at 1/300 = 1,200 ÷ 300 = 4 sq ft (576 sq in)
- Intake at soffits = 2 sq ft (288 sq in) → about 29 round soffit vents or 48 ft of continuous soffit vent
- Exhaust at ridge = 2 sq ft (288 sq in) → about 16 ft of ridge vent (18 sq in/ft)
Common mistakes
- Counting vent cover size as NFA. Louvers cut airflow roughly in half — use the spec-sheet NFA, not the vent's physical size.
- Exhaust without intake. Ridge vents with blocked soffits can pull humid air from the house instead of venting the attic.
- Ignoring the vapor barrier. No vapor barrier under the insulation means 1/150, not 1/300 — that's double the vents.
- Blocking vents with insulation. Baffles at the eaves keep blown-in insulation from covering the soffit intake.
FAQ
1 square foot of net free area per 300 square feet of attic floor area. A 1,200 sq ft attic needs 4 sq ft of total ventilation — 2 sq ft intake at the soffits and 2 sq ft exhaust at the ridge.
For every 300 square feet of attic floor area, provide 1 square foot of net free ventilation area. If there's no vapor barrier over the ceiling, tighten it to 1/150.
Roughly half of your total ventilation goes at the eaves as intake — for a 1,200 sq ft attic that's 2 sq ft of net free area in soffit vents.
Ridge vents provide about 18 square inches of net free area per linear foot. A 40 ft ridge vent delivers about 5 sq ft of exhaust — enough for up to a 1,500 sq ft attic with balanced soffit intake.
1/150 is the safer, higher-performing choice — more airflow, less summer heat and less winter moisture. Use 1/300 only when a proper vapor barrier covers the ceiling.